3. Numbering Schemes

Numbering Schemes

There were many different versions of Strowger type exchanges. The smallest ones, serving small remote villages would handle just a few subscribers. The larger exchanges in urban districts could handle thousands of subscribers. In a small village, there
might be just 50 subscribers and so three digits would be plenty to identify them all. For example, subscribers on a very small rural exchange might be allocated numbers in the range 200-299 - on the final selector, level 2. Of course, numbers of just two digits would have
been enough to cover 50 subscribers, but three digits were used to allow for special codes (Operator, Emergency, Telegrams etc.) and also to separate payphones onto other 'levels'. On larger exchanges, four or five digits were used, allowing a theoretical maximum of 10000 or 100000 subscribers. The range was limited of course because there were no subscribers on levels '0' or '1' as they were reserved for trunk and operator calls respectively.

The Director System

Note : The Director System is considerably more complex than is detailed here so some aspects have been ignored, notably the system of initial digit grouping.

Calls made to other subscribers within the same exchange were obviously routed locally within the exchange however many larger towns had many exchanges, each within the same area. To connect to another exchange, its code could be dialled and the selectors would route accordingly, but that dialling code would have to vary depending on where the call originated from because obviously routes would vary. From the subscriber's point of view, it would be unacceptable to have to dial a different number depending on where you were. To get around this, a set of uniform dialling codes was introduced so that a subscriber could dial
the same exchange from any other exchange always using the same dialling code. Because the actual routing would vary depending on where the call originated from, a piece of equipment called a Translator was introduced. This took the uniform dialling code as the
subscriber dialled it and translated it into the necessary impulse trains so that selectors could be routed accordingly.

The translator was electromechanical of course. The translator also includes 'digit absorbtion' facilities so that if a subscriber dials someone on the same exchange, the exchange code is ignored and the call routed locally. When the subscriber dials the exchange code, the translator cannot start 'translating' until it has all three digits, then it can get to work, but in the meantime, the subscriber may dial the rest of the number. To allow for this, the translator must have Digit Storage facilties so that it can
store the rest of the digits dialled by the subscriber and repeat them to the remote exchange once the connection has been established.

The Mnemonics System

It was thought that people would have problems remembering seven digit numbers (3 exchange + 4 subscriber) so a system of allocating letters to the dial to make area mnemonics was developed. Each exchange was then given a code according to the location, as closely as possible. The original British lettering scheme was as follows :

This 'letter to number' scheme varies between countries and nowadays even between manufacturers, particularly with mobile telephones. The letter 'O' was mapped to the digit '0' in order to avoid confusion. The letters Q and Z were not used in the original scheme to avoid confusion with 'O' and '2'. When the scheme was first devised, the letters were black and the figures in red (all phones were owned
by the Post Office).

It is interesting to note that whilst the authorities of the day considered that people would be unable to remember 7 digit numbers, current proposals in the UK today are to extend the entire numbering scheme to eight digit subscriber numbers. This is already the case in France, where numbers are shown thus : xx xx xx xx.